Booking Ferry from Rijeka to Dubrovnik

by Mark
(GRand Rapids, Michigan, USA)

We are planning to take the ferry from Rijeka to Dubrovnik in september and would like to book a room to sleep in for 2 people. Is there much difference from inside or outside cabin? Should we book the ferry in advance? Also taking the ferry from Split to Ancona, should we book this ferry in advance?

Thanks, Mark